1. Field of the Invention
The present invention relates to a high-frequency discharge plasma generator that is used for suppressing an electrostatic charge (charge-up), etc., on a surface of a substrate when ion beam irradiation is carried out in an ion beam irradiation device that performs ion implantation, etc., by, for example, irradiating the substrate with an ion beam.
2. Description of the Related Art
A plasma generator is disclosed in Japanese Patent Application Laid-open No. 2002-324511 (Paragraphs 0031 to 0038 and FIG. 1) as an example of a high-frequency discharge plasma generator described above used for suppressing an electrostatic charge on a surface of a substrate. The disclosed plasma generator generates a plasma by ionizing a gas with a high-frequency discharge in a plasma generating chamber. As a result, electrons from the plasma are emitted outside the plasma generating chamber through electron emitting holes. In this plasma generator, an inner wall and an antenna of the plasma generating chamber are covered with an insulator to prevent metal contamination produced in sputtering by the plasma and adhering of the conductive sputtered material to the antenna.
The principal object of providing the insulator on the inner wall is to prevent contamination (that is, metallic contamination) of the plasma from occurring. That is, to prevent particles of metal constituting the inner wall being discharged in the plasma from the antenna in sputtering by the plasma.
Alumina, etc., is used as the material of the insulator. An extracting power supply 56 is connected between a plasma electrode 16, which has electron emitting holes, and a target chamber 8. The plasma electrode 16 is made of a conductive material such as carbon. A current that flows through the extracting power supply 56 is called a PFG current Ipfg and is a measure of the electrons that are emitted to the outside through electron emitting holes 18.
The plasma electrode 16 is in contact with a plasma 20 and is operative to ensure an electric potential of the plasma 20. The electric potential of the plasma electrode 16 is set the same as that of a plasma generating chamber 12. When the plasma generating chamber 12 is internally completely covered with the insulator, no conductor is in contact with the plasma 20, no current flows in the plasma 20, and the electrons can hardly be extracted from the plasma 20. However, the plasma electrode 16 can prevent such situations from occurring.
If the plasma generator 10 is driven for a prolonged period (for example, approximately a few hundred hours to a few thousand hours), the PFG current decreases to such an extent so as to be of no use.
If the PFG current Ipfg decreases as described above and neutralization of charge-up of the substrate cannot be performed adequately, the plasma generator has to be removed for clearing the insulating material accumulated on the plasma electrode 16. This results in stoppage of the ion beam irradiation device for maintenance for a long time.
A plasma is a good conductor and by itself is quasi-neutral. Therefore, an electron current lost from the plasma and an ion current are always equal in magnitude. Because a decrease in plasma electrons takes place due to extraction of an electron current from a PFG (PFG current Ipfg), the same amount of ions needs to be lost from the plasma.
Although the ions can obtain the electrons by recombining in the plasma, the electron current lost from the plasma cannot be compensated. An ion current flow is initiated only when the ions collide against the wall, releasing the electrons from the wall.
When the ions collide against the wall, the ions recombine with the electrons at the wall and are converted back into a neutral gas. These electrons are supplied by a PFG power supply 30 through a conductive wall. The PFG power supply 30 also extracts electrons from the PFG, and supplies an amount of electrons to a PFG plasma via the ions that is equal to the amount of electrons that flow into a vacuum chamber. As a result, an outflow current is maintained equal to a feedback current of the power supply.
Thus, for the PFG current Ipfg to flow, the conductive wall must be in contact with the plasma. If the surface of the plasma electrode 16 becomes insulating because of deposition of an insulating material generated due to sputtering, etc., of alumina that is provided on the inner wall of the plasma generating chamber, no PFG current Ipfg will flow.